After recently replacing the heater core in my "parking lot" car (read American), I was discussing the problem with a mechanic friend. It seems that in most recent American cars, the heater control valve which controls hot water flow to the core has been removed. Too many cars were having the heater core corrode through when the owner would shut off the heater in early Spring and not turn it on again till Winter. The water trapped in the heater cannot circulate with the heater control off and also does not get the benefit of the water pump lube and anti-corrosion additives you add (you do, don't you?). BMW 2002 still have heater control valves, so it's a good idea to turn on the heat (not the fan) a few times during the Summer and circulate some water through the heater core.
Author: Ed Vozek